Those familiar with the processor business will be familiar with the term Thermal Design Power (TDP), which refers to the number of watts that a cooling system must dissipate under a maximum real program load. AMD, however, has adopted a different power measurement system to rate its processors.

The new system, dubbed Average CPU Power (ACP), was championed by AMD for the first time with the release of its K10 (Phenom) line. AMD claims ACP is comparable to TDP measurements in Intel's benchmarks and is in fact a superior measuring stick for power consumption. ACP is described as being calculated based on an "average" daily use scenario for desktop users or for a server.

Despite AMD's claims that ACP is directly comparable with TDP, many began to have doubts when AMD struggled to raise the speeds on its Phenom processors. This could indicate, among other things, thermal issues due to high power consumption. With its processors still in the lower half of the 2 GHz range, the AMD documentation has been combed over to evaluate AMD's claims and determine the true story of ACP.

The results are startling. The story is best explained by two versions of a white paper ironically titled "ACP – The Truth About Power Consumption Starts Here," -- the old version can be found here and the new version here. The key information in these papers can be seen in the table below.

Look at those numbers closely. The first thing you'll notice is that TDP is significantly higher than ACP. So when AMD has been comparing its power numbers to Intel chips, using ACP is actually significantly underestimating power consumption. Secondly, note that TDP differed between the two versions of the white paper by as much as 20 W, which is a 21% increase in the case of the Quad-Core Opteron. Thirdly, note that AMD has not increased its ACP estimates, which it emphasizes in bold, despite the TDP clearly rising.

Either the ACP is an arbitrarily measured system, and AMD is changing it at will for its convenience, or AMD's document team failed to update the document properly which one would hope would be the case. There is no other feasible explanation of why a 20 W TDP increase would be accompanied by no increase in ACP. Also, the documents state a theoretical maximum power, which also has not been updated, but has surely increased in reality.

Whitepapers are a very important tool for an electronics firm to inform the public of its products' performance. Glaring inaccuracies such as these certainly reflects negatively on AMD. Furthermore, the admission that ACP can be as much as 20 W lower than the TDP on the same processor, indicates that AMD is being spreading misinformation in using these ACP numbers in its advertising, as Intel uses TDP numbers.

To put this information in perspective, a 3.16 GHz Xeon X5460 from Intel squeaks in at a still weighty 120 W. While AMD has failed to disclose in the white paper what speeds its selected processors were, it is almost surely 3.0 GHz or lower, as 3.0 GHz is the highest speed K10 processor currently demonstrated. The best case scenario is that a 160MHz slower AMD processor consumes 17 more watts. If this was truly the case, it would not be the end of the world, but it would indicate AMD falling behind further in performance.

However, if the samples tested were lower than 3.0 GHz, obviously the picture becomes far worse. And since AMD's 2008 roadmap states that its 2.4 GHz processors will be at 125 W, this is almost certainly the case. So architecture and design advantages aside, you have a chip that is almost a gigahertz slower, at a higher power consumption. Further, you have misrepresentation of your power information, either intentional or unintentional, and erroneous documentation.

AMD is currently suffering through lackluster reception of its Phenom K10 line, and ever-higher-piling losses. The problems have led AMD to try to focus on expanding its K8 platform offerings, instead of addressing the compounding issues of its K10 platform. The heat and power issues covered in this article are just one of many issues with the K10, but have many ramifications -- everything from increased rate of chip failure, to simple higher energy bills at no additional speed benefit. Expect AMD to fix its documents soon, but it will take far longer for AMD to salvage the physical hardware of its K10 platform.

The inconsistencies between these two whitepapers included a table where the Thermal Design Power of AMD's new quad-core Opterons increased, without an increase in the Average CPU Power rating.

Several processor architectures ago, AMD and rival Intel used the same methods for calculating Thermal Design Power with regard to microprocessors. From an engineering standpoint, the TDP represents the amount of power the cooling mechanism for the CPU must dissipate before failure.

AMD and Intel now differ with TDP calculations, and for different reasons. Intel's current architecture, for example, allows the CPU to exceed the TDP rating for a small period of time before the processor throttles its frequency clock in order to reduce the temperature at the processor level. AMD's current-generation processors do not practice this method, and thus AMD intentionally publishes conservative TDP ratings.

AMD's Brent Kerby, author of both whitepapers, explains the inconsistency with timing and the nature of ACP itself. "The measured value of ACP already included the changed TDP values," he explains.

So are the ACP measurements represented in both documents wrong, or mutually exclusive? "No," says Kerby. Even though the maximum thermal envelope has increased by as much as 21% between the two whitepapers, the algorithm for calculating the ACP is not entirely affected by this upperbound.

"When we published the first whitepaper, we had to anticipate TDP changes," adds Kerby. The newest whitepaper, Kerby states, is relevant to both published TDPs. The TDP references in the first document can be replaced with the TDP changes from the second, and in fact should have been.

Kerby could not comment on the specific margins built into the ACP rating, but mentions it's an integral part of the ACP process. His whitepaper details the margin as follows:

The results across the suite of workloads are used to derive the ACP number. The ACP value for each processor power band is representative of the geometric mean for the entire suite of benchmark applications plus a margin based on AMD historical manufacturing experience.​

However, Kerby could confirm that if the TDP were to increase again, the company's ACP values would need to be recalculated.